I2C Question

[junk_no_spam]

Maybe not the correct place to ask this question, but I did see some I2C
threads here. Perhaps someone could direct me to a better format for
this question:

Philips I2C 2.1 spec suggests that full-speed(400KHz) and
high-speed(3.4MHz) devices can exist on the same I2C bus.

So for HS-mode (=3.4MHz device), the clk high/low period is
around ~150nsec.

And, the FS-mode (=400KHz device) has 50nsec glitch rejection filters
on SCL and SDA.

Then how does the FS-mode device reject the HS-mode traffic if the
anti-glitch filter on the FS-mode device passes all of the HS-mode
signaling???

I kind of assumed the anti-glitch filtering on the SCL/SDA
inputs to the FS-mode, would protect the FS-mode device from
HS-mode signals.

Do I need to do some additional digital filtering on the SCL/SDA inputs
to the FS-mode I2C blk in the FS-mode device? And this would be kinda
hard for an asynchronous(no master clk) I2C interface.


What did I miss in the Philips spec?

thanks,
-steve

 

[gabor]

junk_no_spam wrote:

Maybe not the correct place to ask this question, but I did see some I2C
threads here. Perhaps someone could direct me to a better format for
this question:

Philips I2C 2.1 spec suggests that full-speed(400KHz) and
high-speed(3.4MHz) devices can exist on the same I2C bus.

So for HS-mode (=3.4MHz device), the clk high/low period is
around ~150nsec.

And, the FS-mode (=400KHz device) has 50nsec glitch rejection filters
on SCL and SDA.

Then how does the FS-mode device reject the HS-mode traffic if the
anti-glitch filter on the FS-mode device passes all of the HS-mode
signaling???

I kind of assumed the anti-glitch filtering on the SCL/SDA
inputs to the FS-mode, would protect the FS-mode device from
HS-mode signals.

Do I need to do some additional digital filtering on the SCL/SDA inputs
to the FS-mode I2C blk in the FS-mode device? And this would be kinda
hard for an asynchronous(no master clk) I2C interface.


What did I miss in the Philips spec?

thanks,
-steve

Steve,

I think you missed the SDAH / SCLH signals. These are two separate
lines
in addition to SCL and SDA in a mixed system, so the full and
high-speed
modes can only be mixed in a 4-wire system. With 2 wires you're stuck
with one or the other.

Regards,
Gabor